1. Field of the Invention
The present invention relates to a thermal head with a thin Ta-Si-O film as a resistive layer.
2. Description of the Prior Art
A thermal head with a resistive layer in the form of a thin film is used in an electrical thermal conversion output unit used in printing heat-sensitive paper. A conventional thermal head is manufactured as follows. A high-resistance substrate of, for example, alumina, is prepared one surface of which is covered with a thin glass layer. A resistive layer and lead layers for energizing the resistive layer are formed on the thin glass layer, in accordance with a known thin film fabrication technique. A protective film is formed on the resistive layer and lead layers as needed. Then, reverse-flow preventive elements such as transistors or diodes are arranged to provide connection between common leads and individual leads. A heat sink can be formed to provide a heat dissipation effect. Typical materials of which the resistive layer in a thermal head can be made are Ta-N, NiCr, or Ta-SiO.sub.2.
In a thermal head having a resistive layer made of one of the above materials, a pulse current is supplied from an external power source to the resistive layer and heat generated by the resistive layer is conducted to heat-sensitive paper so as to change white into black and thereby perform printing. FIG. 4 is a sectional view of a conventional thermal head, illustrating the direction in which heat generated by the resistive layer is conducted. When current flows through resistive layer 3 upon application of a pulse voltage thereto, resistor 3 is heated by joules heat corresponding to the current in the resistor. Heat generated by resistor 3 is divided into heat flow q.sub.P, conducted to heat-sensitive paper (not shown) through an antiwear protective layer 6, heat flow q.sub.B, conducted to glazing layer 2 and alumina substrate 1, and heat flow q.sub.S, conducted to lead layers 5. In general, thermal efficiency of heating dots in the thermal head is good as flow q.sub.P is increased and flows q.sub.S and q.sub.B are decreased. In a conventional thermal head, flow q.sub.P can be as undesirably low as about 20%. This level of thermal efficiency cannot provide good printing.
U.S. Pat. No. 4,105,892, issued to Tashiro et al., describes a thin amorphous film of Ta-Si-O, containing 7 to 53 mol% SiO.sub.2. Tashiro et al. describe reproducibility and uniformity of the resistor but do not describe any improvements in the thermal efficiency of the heating dots.
Japanese Patent Disclosure No. 57-61581, describes a target used in forming a thin resistive layer film. The target is a sintered body obtained by sintering tantalum and a silicon dioxide. The content of tantalum is 63 to 55 mol%. Takeno et al. describe reproducibility and uniformity of the resistor but do not describe the heating efficiency of heating dots.